Human perception of images relates to visual cues within images. One example of an image cue is bokeh (also referred to as lens-blur) in the foreground and background of an image. The presence of bokeh indicates a short depth of field in a photo, which in turn perceptually indicates the scale of a photographed object to a viewer, even if the object in the photograph is not familiar to that viewer. Manipulating bokeh in an image, for example by a direct blurring or sharpening of different parts of the image, can change how the size and distance of objects in the image are perceived and how the image is interpreted by a viewer, separate to a viewer's ability to see the blurring and sharpening. A key understanding is that what is perceived by the viewer, is different to the change made to the image. A popular feature in compact cameras and image processing software, is the “tilt-shift” or “miniature” effect, in which artificial depth of field is introduced into an image by selective blurring of an image, which results in objects in photographs being perceived by the viewer to be much smaller than their actual size.
“Rittai-kan” represents “a feeling of 3D”, or a “sense of depth” of the content of a 2D image. It is an example of a perceived property of an image. Rittai-kan is highly-valued perceptual property by Japanese professional photographers. Some aspects of rittai-kan are compositional and therefore set at the time that an image is captured, and cannot easily be adjusted later.
Other aspects are may be adjusted after capture. Rittai-kan is a perception, affected by image composition and appearance, not tied to any one property or modification of an image.
FIG. 2A shows an image with a small level of rittai-kan, whereas FIG. 2B shows an image with a high level of rittai-kan. FIG. 2A shows an image 200, with a flower 210, in front of a background 203 with a horizon line 205. The flower has a leaf 215 partly occluded by a triangle 220. The triangle has visible edges 225.
FIG. 2B shows a second image 230, which is the result of applying non-photorealistic processes to the image 200 in FIG. 2B in order to give a greater perception of depth to the image. The flower 240 appears as before, but changes have been introduced to its leaf 245 and the background 233. To create a “pop-out” effect, part of the background 265 has been blacked-out, and a frame 260 has been drawn around the remaining background 233, passing behind parts of the flower 240. To change the apparent order of objects in the image, the flower's leaf 245 has been inpainted to cover the triangle 250. To give the flower 240 the appearance of floating above each of the remaining background 233, the introduced background 265 and frame 260, a drop-shadow 248 with the same shape as the flower 240 has been drawn behind the flower but over those elements.
The example manipulations in FIG. 2B are overt, they change how the relative depths and distances in the image are perceived by a viewer, even though the image is still two-dimensional. The example manipulations in FIG. 2b are examples of “monocular depth cues”. A number of monocular depth cues are known, and these may be applied to a 2D image to change the rittai-kan (perception of depth) in the image while still outputting a 2D image. Monocular depth cues affecting rittai-kan include, for example, perspective lines, aerial perspective (also known as distance-haze), texture gradients, lighting and shading, and defocus blur. A number of non-photorealistic manipulations are currently known and applied in software.
Some methods for manipulating images to introduce monocular cues and combining the monocular cues to increase perceived depth of an existing image are known. Achieving such edits in a photorealistic manner is particularly difficult. Typically, the skill to make photorealistic edits, in order to change how an image is perceived, is in the domain of fine-arts and skilled artists. Often, the development of this skill is intuitive or non-technical. Learning the art of incorporating different perceptual aspects into an image is time-consuming and inexact. A user may know that they wish to increase the rittai-kan of a realistic image, but may not have the skills, experience or time to achieve a suitable natural effect. There exists a need for an improved approach to modification of rittai-kan in an image.